I'm not quite sure, but I think your explanation of No. 54 is incorrect.

You said,

“D” deletes the current line (line that matched “/regexp/”) from pattern space and finally “h” gets executed again, that puts the contents of pattern space into hold buffer.

But, "D" command deletes the current line and start next cycle immediately. So last "h" command will not execute. But, previous "x" command stored the current line (not the next line) into the hold buffer already. And pattern space has the next line because of the "D" command.